Remove the shingles covering the rotted section of roof by prying the nails out of the wood beneath. Remove the roofing felt beneath the shingles as well.
Uncover an area slightly larger than the damaged area of decking.
Cut out the damaged roof decking using a saw. Make sure you remove all of the rotted wood and that the edges of the area you uncover are supported by a rafter.
Be careful not to cut into the rafters beneath as you cut.
Measure all four sides of the area that you’ve cut out and cut the same sized piece out of a new piece of plywood.
Fit the new piece into the cut-out area. If it does not fit flush to the cut area, trim the edges using a saw in increments until it does.
Secure the new piece of decking by nailing or drilling screws through the decking into the rafters or studs beneath. Use either roofing nails or 2- to 2 1/2-inch deck screws.
Roof decks are typically secured using only enough fasteners to keep the plywood from sliding off.
Make sure each nail or screw meets and goes into a rafter to ensure proper fastening. To check, shine a light into the attic space where the roof meets your rafters.
If you see the nails or screws shining, you may have to add a few more to ensure it is securely fastened.
Tape the seams of the plywood patch using modified bitumen tape. Make sure all seams are covered.
Cut a piece of roofing felt the same size as the patched area and install so that each piece overlaps the previous by four to six inches.
Install new shingles over the repaired section of roof deck.